Test cleanup: Remove an obsolete fixture suite and add .hidden to a few tests

[stmontgomery]

A few small cleanups to the project's unit tests:

• Remove the obsolete fixture suite type IndependentlyRunnableTests. The test which used to reference it has been deleted.
• Add the .hidden trait to a few eligible @Test functions.

Checklist:

• Code and documentation should follow the style of the Style Guide.
• If public symbols are renamed or modified, DocC references should be updated.

[stmontgomery]

@swift-ci please test

[grynspan]

To be clear, this test doesn't ever compile. It's useful when testing exit tests to verify what happens if you attempt to close over some state.

[stmontgomery]

Yeah, understood. Despite being compiled out, without this trait it can still be included in the indexer's test discovery.